Research news

Study of two large crystal skulls in the collections of the British Museum and the Smithsonian Institution

British Museum skull

The life-size carving of a human skull in the British Museum collection was made from a single block of quartz crystal (a clear colourless variety of quartz known as rock crystal). According to Museum records, the skull was acquired in 1897 from Tiffany and Co., New York, through Mr George Frederick Kunz.

In one of his numerous publications, Kunz claims that the skull was brought from Mexico by a Spanish officer before the French occupation (1. See references at the bottom of the page). It was sold to an English collector and acquired at his death by Eugène Boban, a French antiquities dealer, later becoming the property of Tiffany and Co.

They were also carved in relief in basalt or limestone as architectural elements, as can be seen in the monumental circular stone relief discovered at the Aztec Templo Mayor in what is now Mexico City.

However, the authenticity of skulls made of quartz crystal soon came to be questioned. Although some are said to be examples of colonial Mexican art for use in churches, perhaps as bases for crucifixes, they may be among the large quantities of forgeries produced during the second half of the nineteenth century, when interest in collecting ancient artefacts from Mexico was at its height in both the United States and Europe. Some of these pieces made their way into museum and private collections.

Staff in the Department of Scientific Research at the British Museum examined the British Museum skull several times between 1950 and 1990 (3). Observations made with a binocular microscope suggested that the techniques of carving were probably atypical of pre-Columbian times. Also, the large piece of rock crystal used for the skull was thought to have come from Brazil, an area far beyond ancient Mexican trade links.

Smithsonian skull

An increasing number of large and small quartz skulls have become known, particularly in recent decades. None has ever been reported from well-documented official archaeological excavations.

In 1992, almost a century after the crystal skull was acquired by the British Museum, a particularly large white (or milky) quartz skull with a hollow cranium (right) was sent anonymously to the Smithsonian Institution, Washington D.C. An accompanying note said the object was bought in Mexico City in 1960. The carving, like the British Museum skull, is stylistically somewhat anomalous when compared with ancient Mesoamerican depictions. For example on both skulls, the rigid linearity of features representing teeth contrasts with the more precise execution of teeth on pre-Columbian artefacts.

The arrival of the white quartz skull led to a study of archival documents concerned with the early history and acquisition of several crystal skulls in museum collections. It became apparent that not only had the dealer, Eugène Boban, owned the British Museum skull (as alluded to above), he had previously also been involved in the sale of three other rock crystal skulls, one which is around 11 cm high and two small ones (which are less than five cm high), currently in the Musée du Quai Branly, Paris (4,5,6).

Collaborative study

In 1996, a collaborative programme of authenticity studies was set up between the British Museum, the Smithsonian Institution and the Department of Earth Sciences and Geography at Kingston University, Surrey.

Small skulls carved from rock crystal have perhaps attracted less public attention than larger examples, and the investigations were focused on the origin of the large skulls in the two national museums. Because no reliable means of directly dating stone objects was available, the aim of the project was to answer three questions. How were the skulls carved? Where did the large pieces of quartz originate from? What is known about the early history of the skulls?

A number of rock crystal artefacts of undisputed origin are also known. They include beads and ear flares. The opportunity was taken to compare the two large skulls with several other rock crystal objects from well-documented excavations in Mexico City and Oaxaca, Mexico, including the goblet shown here, which is 8.8 cm tall and is the largest documented rock crystal artefact to have been recovered from a pre-Columbian site. The goblet is now in the collection of the Museo de las Culturas de Oaxaca.

Technology of carving

An approach developed at the British Museum for investigating carving methods was adopted for the study of skulls. This enables the use of tools and techniques to be identified from the fine detail of the carved features or ‘tool marks’ preserved on hard stone objects (7). In the investigation of the skulls, the faint tool marks remaining on the highly polished surface of the British Museum skull and the pitted matt surface of the Smithsonian skull were examined under a microscope. Moulds were also made of these tool marks using special silicone dental ‘wax’ and these were examined at high magnification in a scanning electron microscope (SEM).

The tool marks on the skulls were compared to the tool marks remaining on genuine pre-Colombian rock crystal objects. The regular characteristics seen on both skulls showed they were mainly worked with rotary wheels in conjunction with very hard abrasives. In the SEM image shown below, the curvature along the moulded teeth shows they were cut using a wheel.

In the SEM image (above right) of a mould taken from the teeth of the British Museum skull, the curvature along the end of the mouth indicates that, before the skull was polished, this narrow feature was cut using a wheel. The teeth were worked in two stages: the curvature along the upper narrow cut (on the mould) shows that, after polishing, a cutting wheel was again used to emphasize and deepen the existing shallow features in the skull.

The regular characteristics of the tool marks seen on the skulls show that they were both extensively worked with rotary wheels and hard abrasives. The characteristics of the tool marks on the skulls contrast with those of tool marks seen on pre-Columbian pieces, which were carved with hand-held tools, as can be seen from a painting in the Aztec Codex Mendoza in the collection of the Bodleian Library, Oxford.

Rotary cutting wheels were not introduced to stone workshops in Mexico until after the Spanish conquest of Mexico in 1521. The skulls therefore cannot be of Aztec manufacture.

A further clue to the date of manufacture of the Smithsonian skull was provided by a minute deposit remaining in a cavity in the surface of the skull. This deposit was identified by x-ray diffraction analysis as the twentieth-century, synthetic abrasive known as carborundum (silicon carbide). The pitted white quartz surfaces of the Smithsonian skull were probably worked with wheels made of a bonded material, such as silicon carbide grit in a ceramic matrix, which were first used in stone workshops around the mid twentieth century. The Smithsonian crystal skull appears to have been made shortly before it was bought in Mexico City in 1960. White quartz is of relatively common occurrence and sources of the material are known in Mexico and the USA.

The source of quartz used for the British Museum skull

The large clear quartz crystal used for the British Museum skull would have been obtained from a special source and to investigate the provenance of the geological source, the solid inclusions and numerous fluid inclusions in the quartz were examined. As quartz crystallises from a fluid state, fragments of the surrounding minerals and rocks often become trapped inside. Once cool, the composition and characteristics of inclusions provide a fingerprint of the geological environment and possible source of the material (8, 9).

At high magnification, it can be seen that a rounded vapour bubble formed in each of the liquid inclusions as the quartz cooled. The more or less constant proportion of liquid to vapour in the inclusions throughout the skull and other characteristics indicate that the quartz formed in a moderate temperature mesothermal environment.

The solid inclusions near the base of the skull consist of small green crystals and sometimes have distinctive planar partings in worm-like stacks, as indicated in the photograph below. Using Raman spectroscopy, the green inclusions were shown to be an iron-rich chlorite. These minerals are found in mesothermal metamorphic greenstone environments. Sources of this type are not found in Mexico or within the ancient Mexican trade network.

Furthermore, the distinctive worm-like shape of the chlorite inclusions in the British Museum skull is typical of rock crystal from Brazil and Madagascar. The well-known Brazilian sources of quartz were first exploited by German settlers in the 1930s and sent back to Germany for cutting. Rock crystal was first imported from Madagascar by the French in the late eighteenth century, both for use in their workshops and distribution in Europe. However, it is unlikely that large quartz crystals suitable for the British Museum skull would have been available until the final decades of the nineteenth century.

Early history of the British Museum skull

A production date in the second half of the nineteenth century (and in Europe) for the British Museum skull is supported by the historical evidence, which indicates that the carving was acquired by the French antiquities dealer, Eugène Boban between 1878 and 1881, when he was based in Paris. Having failed to sell the it in Paris, Boban offered the carving to the Museo Nacional de Mexico in 1885. Here it was rejected as a modern European artefact and Boban was denounced as a fraud. Nevertheless the following year, Boban sold the crystal skull to the New York jewellers Tiffany and Co, from whom the British Museum acquired the carving more than a decade later.

Summary

The results of this programme of research into carving technology, sources of quartz and early history combine to demonstrate that the life-size rock crystal skull in the British Museum and the larger white quartz skull in the Smithsonian Institution are not ancient, but are of relatively modern manufacture. The results are published online by the Journal of Archaeological Science: ‘The origin of two purportedly pre-Columbian Mexican crystal skulls’, Journal of Archaeological Science (2008) by M. Sax, J.M. Walsh, I.C. Freestone, A.H. Rankin and N.D. Meeks.

Whatever their origin, the spectacular appearance of crystal skulls continues to fascinate as much as when they first appeared during the second half of the nineteenth century.

This interest continues today - the new film Indiana Jones and the Kingdom of the Crystal Skull explores other avenues of their story.